Charged colloidal particles, when suspended in water, form a stable dispersion due to interparticle coulomb repulsion forces. The property of structural ordering in such dispersions has been exploited in making devices such as narrow band optical rejection filters. The ordering phenomena in such colloidal suspensions has been useful in spectroscopy and Bragg diffraction techniques. See, for example, U.S. Pat. No. 4,627,689. It has been found that mesoscopic, crystalline structures can have many practical applications as optical filters in military, space, medical and research uses. In many such instances, it is necessary or desirable to filter narrow bands of selected wavelengths from a broader spectrum of incident radiation.
U.S. Pat. No. 4,627,689 of Asher discloses a linear crystalline colloidal narrow band radiation filter which is made by forming a highly ordered crystalline colloidal structure within a container. The crystalline colloidal structure is formed by dispersing the ionized particles, for example, polystyrene particles, within an appropriate solvent.
A related disclosure was made in U.S. Pat. No. 4,632,517 of Asher which discloses another crystalline colloidal narrow band radiation filter application. U.S. Pat. No. 4,632,517 forms the basis for a mechanically simple and highly efficient monochromator. It has application in improved systems for investigating Raman or emission spectra of selected sample materials. Both of the aforementioned patents disclose structures that can be used to diffract a narrow band of radiation from a broader band of radiation.
A solid filter and method of making a solid filter from an ordered dispersion of particles within a medium is disclosed in U.S. Pat. No. 5,281,370 of Asher. That patent discloses a filter which is capable of Bragg diffracting narrow bands of radiation. It is a solid filter which has many practical applications.
An optical filter was also disclosed in U.S. Pat. No. 4,548,473. The filter comprises a first substance substantially transparent to light within a select frequency range and having a first index of refraction. The filter also includes a second substance which has at least one resonance frequency within the first frequency range and a second index of refraction which is substantially the same as the first index of refraction at all of the frequencies within the first frequency range except for frequencies near the resonance frequency. This device is based upon resonance scattering by a disordered sample. The device is only a passive device meaning that the index of refraction is not considered to depend upon the incident intensity or time.
U.S. Pat. No. 3,620,597 discloses a device which is capable of acting as a nonlinear absorber of substantially all radiant energy in excess of a predetermined intensity. The mechanism utilized by the device is distinct from that of the present invention.
U.S. Pat. No. 4,832,466 is directed to an optical element including a modulating liquid layer composed of a solvent containing a soluble polymer. The device requires polymers to precipitate from solution due to temperature changes. This is not required by the present invention.
U.S. Pat. No. 4,648,686 discloses an optical switch array which utilizes the temperature dependent characteristics of the index of refraction of a crystalline material, however, the device is limited to being used for switching in a waveguide. Other switches for use in waveguides were disclosed in U.S. Pat. Nos. 4,828,362 and 4,938,557.
U.S. Pat. No. 4,268,413 provides devices having the property of reversibly variable temperature-light absorbance. The device is said to be usable in temperature-measuring devices, slippery ice warning devices and the like.
U.S. Pat. No. 5,452,123 discloses a nonlinear optical device and method for making the same. The method includes making a solid or crystalline colloidal ordered dispersion of charged particles within a medium and introducing into the particles or the medium a radiation responsive component which, when impinged with radiation at a critical intensity, causes a change in the refractive index of the particles in either the ordered dispersion, the medium or both.
U.S. Pat. Nos. 5,368,781 and 5,266,238 are directed to tunable, narrow band radiation filters comprising a crystalline colloidal array of charged particles fixed in a hydrogel film. Methods for filtering incident radiation using these filters are also disclosed.
U.S. Pat. No. 4,720,355 is directed to a non-linear optical medium having a xe2x80x9chostxe2x80x9d thermoplastic polymer which contains a xe2x80x9cguestxe2x80x9d organic component; the organic component has a charge asymmetric electronic stricture and exhibits non-linear optical response.
U.S. Pat. Nos. 5,330,685, 5,338,492 and 5,342,552 all provide narrow band radiation filters comprising a CCA of charged particles in a polymeric hydrogel.
U.S. Pat. Nos. 6,165,389; 6; 6,094,273 and 6,014,246 are directed to devices which comprise mesoscopically periodic materials that combine crystalline colloidal array self-assembly with the temperature and/or pressure induced volume phase transitions of various materials. The devices are useful as optical switches, optical limiters, optical filters, display devices, and processing elements as well as membrane filters.
Despite the above, there remains a need for devices which permit a simplified means of writing and erasing data. Such devices are capable of a photoreversible shift in diffracted light and are useful as optical switching devices. The devices of the present invention fulfill this need by permitting the possibility to write with ultraviolet light and erase with visible light.
The present invention provides devices useful, inter alia, as optical switches, display devices and memory storage devices that are capable of a photoreversible shift in diffracted light.
The devices comprise novel mesoscopically periodic materials which comprise crystalline colloidal arrays (CCA) polymerized in a hydrogel and at least one photosensitive component attached to the polymerized CCA. Preferably, the photosensitive component is a photochromic molecule, for example, a derivative of azobenzene. The periodic materials comprising these photoswitchable CCA have a periodicity that can be reversibly switched with ultraviolet (UV) and visible light. The result is a polymerized CCA (PCCA) which is photoresponsive and which undergoes a continuous volume change upon irradiation. The devices additionally permit the possibility to write with ultraviolet light and erase with visible light, or vice versa.
The present invention also relates to a new method for functionalizing PCCA. In addition, the present invention relates to a new azobenzene derivative which has improved water solubility and which maintains its photochemical properties. The azobenzene derivative (Formula VII) of the present invention may be prepared according to the method described in Example 4 below. 
The azobenzene derivative of the present invention has improved water solubility characteristics than non-derivatized azobenzene. In addition, it retains similar photochemical properties as non-derivatized azobenzene.
Methods for making the devices of the present invention are also provided. Generally these methods involve creating a crystalline colloidal array, using highly charged monodisperse colloidal particles that self-assemble into crystalline colloidal arrays (CCA), and polymerizing a hydrogel network around the CCAs to form PCCAs which permanently locks in the ordering of the CCAs.
In one embodiment of the invention, the PCCA is functionalized such that the photosensitive component may be attached to a PCCA. In a particularly preferred embodiment of the invention, the PCCA is functionalized by the addition of glycidyl methacrylate to introduce reactive epoxide groups. Attachment of a photosensitive component to the reactive PCCA is accomplished through the reaction of reactive groups of the component with the epoxide groups of the PCCA. For example, the epoxide groups inside the hydrogel may be reacted with primary amino groups on an azobenzene derivative. The resultant photosensitive PCCA comprising at least one photosensitive component can undergo a reversible change in volume in response to irradiation. Other photosensitive components that undergo a change in volume in response to irradiation may also be used, such as, inter alia, spiropyranes, spiroxazines, fulgides, diarylethenes, benzothiazolium styryl dyes, and triphenylmethane derivatives.
The devices of this invention can form the basis for mechanically simple and highly efficient optical switches, display devices, and memory storage devices which are useful for computer applications. Overall, the devices can be used with any product in which the disclosed characteristics are desirable.
It is an object of the present invention to provide an optical switching device which can operate to Bragg diffract certain wavelength bands of incident light.
It is a further object of the present invention to provide an optical switching device that undergoes a reversible change in volume upon irradiation.
It is another object of the invention to provide a device that functions as a display device.
In addition, it is an object of the present invention to provide a device that functions as a memory storage device.
It is yet a further object of the present invention to provide a device that can write and erase. For example, but not by way of limitation, the devices of the present invention can write with UV light and erase with visible light.